Antibodies to sperm have been implicated in human infertility, and the deliberate immunization of animals with sperm or mature testis extracts has resulted in a significant inhibition of fertility. Accordingly, researchers have actively pursued the study of sperm antigens in the hopes of identifying a germ cell specific antigen that can be used as an immunogen in a contraceptive vaccine. The approach of identifying gamete specific antigens has the advantage over other approaches, such as the HCG vaccine, of being a pre-fertilization vaccine--one which induces immunity which blocks fertilization as opposed to attacking the early embryo.
A safe and effective contraceptive vaccine would be a highly desirable method of birth control because a single injection or only a very few injections could provide antifertility activity in a human female for several years. However, until relatively recent biotechnologic and immunologic advances, very few antigens suitable for such antifertility vaccines had been identified and purified, especially in humans. Further, human proteins were significantly more difficult and expensive to produce than most viral or bacterial proteins, and were not as immunogenic. The emergence of hybridoma and recombinant DNA technology has provided the possibility of identifying germ cell specific antigens and mass producing the human form of such protein antigens for study and potential use in new vaccines.
Anderson and Alexander, Fertility and Sterility, 40:557-571 (1983) discusses the general application of genetic engineering and monoclonal antibody technology to developing antifertility vaccines. It also discusses some of the candidates for such vaccines, including the sperm antigens protamine, lactate dehydrogenase-C.sub.4 (LDH-C.sub.4), RSA-1, acrosin, and hyaluronidase. The authors state that LDH-C.sub.4 has been purified and amino acid sequence information is available. They further state that monoclonal antibodies (MABs) have been developed to it. The authors also state on page 561 that (1) sperm plasma membrane autoantigens provide the best targets for the effects of antifertility antibodies and (2) antigens bound to the inner acrosomal membrane, such as acrosin and hyaluronidase, appear to be poor candidates.
Over the last several years, many different monoclonal antibodies have been made to human and other animal sperm antigens. For example, Lee et al., Journal of Reproductive Immunology, 4:173-181 (1982), incorporated herein by reference, discloses mouse MABs that react with antigens localized in the acrosomal region of human sperm. Such antigens are apparently on the surface of the sperm, and they have a molecular weight of about 10,000.
Another example is a mouse MAB, designated C11H, to an acrosomal antigen. See Kallajoki and Souminen, International Journal of Andrology, 7:283-296 (1984), Kallajoki et al., International Journal of Andrology, 9:181-194 (1986), and Salonen and Kallajoki, International Journal of Andrology, 10:731-739 (1987), all of which are incorporated herein by reference. The 1984 paper discloses the preparation of C11H and that it recognized an antigen of 50,000 molecular weight as well as other components of 24,000-34,000 molecular weight. The antigen was found in the sperm of humans and certain animals. The authors indicated that they believed the antigen to be acrosin, and they stated that they did not know whether it was in the acrosome or within the acrosomal membranes. The 1986 paper provides further information about the antibody and antigen. The authors state that the antibody reacted with acrosin and further that acrosin is in the acrosomal matrix. They suggested that acrosin is almost totally liberated during the acrosome reaction. They further state that it reacted against a 50 Kd antigen and several others in the 24-34 Kd range. Finally, they state that the MAB can be used to screen for acrosome-reacted sperm. The 1987 paper discloses experiments in which C11H inhibited sperm penetration of zona-free hamster eggs.
Huneau et al., International Journal of Andrology 11:13-24 (1987), incorporated herein by reference, discloses a mouse MAB, designated a-HS 1E.1, which reacts with human sperm in the equitorial region of the acrosomal membrane. FIG. 2 in the paper indicates that the MAB reacts with the outer acrosomal membrane. The corresponding antigen has a molecular weight equal to or greater than 53 Kd.
Such antibodies provided the possibility, at least in theory, of identifying, isolating, and characterizing gamete cell specific antigens that might be useful in a contraceptive vaccine. However, the efforts to date have been disappointing.
Anderson et al., Journal of Reproductive Immunology, 10:231-257 (1987), incorporated herein by reference, discloses a multi-laboratory effort sponsored by the World Health Organization (WHO) to evaluate 66 different mouse MABs that react with human sperm. Of the 66, only 3 reacted with antigens that looked like good candidates for a contraceptive vaccine. One of these monoclonal antibodies, designated MHS-10, showed strong human sperm and testicular germ cell reactivity and a lack of cross-reactivity with many other adult tissues. The antibody inhibited sperm/egg binding in the hamster egg penetration test. The authors also stated that MHS-10 bound to a human sperm surface antigen and that it reacted with a family of antigens with molecular weight between 14,000 and 30,000.
The authors disclosed various difficulties in evaluating this MAB and the other MABs. They concluded from their evaluation of all of the antibodies that the mouse monoclonal antibody approach is not efficient for the identification of human reproductive tissue-specific antigens and further that the immunohistological data and the "surprising cross-reactivity of [most of] the MABs with non-reproductive tissues" underline the necessity for extensive immunohistologic testing of new MABs by qualified immunopathology groups.
At least one recent study continues to reflect the conventional wisdom that an antifertility antigen should appear on the sperm surface. Primakoff et al., Nature, 355:543-546 (1988), incorporated herein by reference, reports an affinity-purified guinea pig sperm protein, designated PH-20, which was used as an immunogen to prevent conception in male and female guinea pigs. The antigen, which has a molecular weight of 64,000, is present on both the plasma membrane and inner acrosomal membrane of guinea pig sperm. In the last paragraph of the article, the authors state that the high contraceptive effectiveness of the antigen depends upon several specific properties, including its presence on the sperm surface. They further state that a human functional analog of PH-20 would be a candidate for an effective contraceptive immunogen.
Herr et al., Journal of Andrology, 9:42 (1988) is an abstract that reports further data on the antigen identified by MHS-10. In particular, it discloses that the antigen is localized to acrosome-shaped structures in the human sperm and that the peptide has 7 major isoforms with a molecular weight of 22-38 kD.
MSH-10 and its corresponding antigen, human acrosomal sperm antigen 10 (SP-10), have now been substantially purified and characterized by the inventors. The inventors have surprisingly discovered a class of intra-acrosomal human sperm antigens that, contrary to the conventional wisdom, may have antifertility activity when used as an immunogen in a contraceptive vaccine for human females. The inventors have also found the SP-10 antigen in other primates. The inventors have isolated the cDNA coding for the SP-10 antigen, which permits the use of genetic engineering methods for making the antigens in a form useful as a immunogen in a vaccine.